The over-all objective is to deduce the genetic mechanisms which control the fundamental body segmentation plan of a higher organism; namely a division of the body into head, thoracic and abdominal segments. The bithorax gene complex in Drosophila will be used for this purpose since extensive studies already have shown that it controls the major pathways by which certain thoracic, abdominal and possibly head patterns of differentiation are brought about. The proposal is to test the hypothesis that another gene, Polycomb, in the left arm of the third chromosome codes for a repressor of the bithorax gene complex. Homozygous deficiencies for the region containing Pc will be synthesized. It is anticipated that they will die in the embryonic stage with striking transformations characteristic of, but more extreme than, the homozygous Polycomb mutant; namely, transformations of thoracic and abdominal segments toward the eight-abdominal type of segment. A strong maternal effect of the number of doses of the bithorax complex on the expression of the Polycomb mutant phenotype will be investigated to determine the degree of autonomy as well as the temperature stability of that effect. Revertants of regulatory mutants of the bithorax complex will be analyzed to determine the number and properties of structural-like genes within the bithorax complex that are responsible for abdominal segmental differentiation.